There is known a network system in which a network device as a primary device (so called “master”) controls other network devices as secondary devices (so called “slaves”) (Refer to Patent document 1).                Patent document 1: JP-2000-250850 A        
In such a system, after receiving an instruction from the primary device, the secondary device transmits to the primary device a response that a process has been executed according to the instruction. After receiving the response, the primary device continues subsequent processes.
When the response from the secondary device does not reach the primary device within a predetermined period because of a certain reason, the primary device generally issues the same instruction to the secondary device again to urge the secondary device to execute the process and respond.
For instance, a response from the secondary device may not reach the primary device because of a reason such as a trouble in the communication channel. In this case, in spite of having executed the process based on the previous instruction, the secondary device may execute the same process again in response to the same instruction from the primary device.
This consumes the resource of the secondary device unnecessarily; further, the response from the secondary device can reach the primary device after the secondary device has executed the same process in multiple times. This results in worsening the response of the secondary device to the instruction from the primary device.